It is common practice in making paper and board to add particulate materials, for example mineral pigments, for filling and loading purposes to the pulp slurry from which the paper or board is made. The common mineral fillers are considerably cheaper than pulp and therefore reduce the cost of the paper or board product; moreover, fillers may be used to improve such properties as the brightness, opacity, handle, ink receptivity and print clarity of the product. However, fillers almost invariably reduce the strength of the product. Furthermore, there is a tendency for filler particles to be lost into the water which drains from the fibrous web formed from the pulp slurry, although the amount of filler that is lost will depend upon many factors such as the particle size and the specific gravity of the filler.
The loss of filler can be reduced by the addition of a retention aid. Certain retention aids act to neutralize the negative charges that develop on the surfaces of the filler particles and fibers and thereby to encourage coflocculation of the filler and fibers, such charge-biasing retention aids including polymeric flocculating agents having a molecular weight of the order of 10.sup.3 to 10.sup.5 and which contain amine or quaternary ammonium groups, for example polyamide-epichlorohydrin condensates or poly(dimethyldiallyammonium chloride). More effective as retention aids, however, are polymeric flocculating agents having higher molecular weights, usually of the order of 10.sup.6 to 10.sup.7, amongst which the ionic polymers, especially ionic copolymers, of acrylamide are commonly used, although polyethylene-imines and vinylpyridine polymers are also effective. The high molecular weight polymers may be referred to as "bridging" polymers, since they encourage flocculation by forming molecular bridges between particles to which they are adopted. The use of fillers and retention aids in papermaking is described, for example, in the articles entitled "Paper" and "Papermaking Additives" in the Kirk-Othmer Encyclopedia of Chemical Technology, third edition, volume 16, pages 768 to 825. Interesting flocculations, which may be used in papermaking, e.g. in the presence of cationic starch, were recently disclosed in EP-A-0,172,723. The teaching of these documents is incorporated herein by reference.
U.S. Pat. No. 2,027,090 (Carter) discloses a method of incorporating a substance into paper or the like by dispersing the substance in a continuous phase capable of coagulating to a firm gel, an aqueous solution of viscose cellulose being a particular example of such a phase. The gel is subdivided into particles (as in the papermaking beater). Furthermore, fibers are bound into the gel in such a manner as to protrude from the said gel particles; preferably such fibers are initially introduced into and dispersed throughout the liquid used to make the colloid dispersion which constitutes the said continuous phase. The gel particles are described and illustrated as enveloping, encysting or enclosing the particles of dispersed substance. Carter's method is suitable for incorporating into paper sticky or gummy substances such as a phenol-formaldehyde condensation product, although mineral fillers are mentioned in passing. The said fibers (which serve as "anchors" for the gel particles) ar preferably the same as those of papermaking pulp stock, although rayon and asbestos fibers are also specifically mentioned.
In order to achieve a high filler content whilst maintaining satisfactory strength properties, in particular tensile strength and burst strength, it has been proposed to employ a preflocculated filler composition, that is to say a suspension of filler to which a flocculating agent, in particular a high-molecular-weight synthetic polymer, is added before the filler is incorporated into the papermaking stock (see M. C. Riddell et al., Paper Technology 17(2), 76 (1976) and British Patent Specification No. 1,522,243, the teaching in which is incorporated herein by reference).
It has been found that the incorporation of filler into paper and other fibrous sheet products by conventional procedures gives rise to products having a poor tear strength, with difficulty in maintaining good formation and adequate tensile properties.